1. Field of the Invention
The present invention relates to an image forming apparatus such as a multi function peripheral or a plotter that includes at least two of the following, namely, an optical scanning apparatus that scans a scanning surface, an optical writing apparatus that includes the optical scanning apparatus, a copying machine that includes the optical writing apparatus, a printer, and a fax machine.
2. Description of the Related Art
Optical scanning apparatuses are widely known in relation to digital copying apparatuses and laser printers. Optical scanning apparatuses employ scanning optical system, which involves focusing optical beams deflected by an optical deflector to form a beam spot on a scanning surface.
In an optical writing apparatus or an image forming apparatus that includes an optical scanning apparatus, there is a tendency for the optical performance to deteriorate due to reduced tolerance of the optical element, expansion/contraction of the optical element due to temperature variation, etc. A stable optical performance is thus desirable alongside a small beam spot diameter.
A stable optical performance can be attained by including an adjustment mechanism in the optical element. However, this method is neither cost-effective nor space-effective.
Stability particularly is sought with regard to the position of the beam spot of different colors in a main scanning direction. A multi-color image clearly shows degradation if the beam spots of all the colors do not form at the same spot.
The position of the beam spot in the main scanning direction can be stabilized, by adjusting the write timing. Synchronization detection method is well known as a method for electrically adjusting a write-start timing and involves providing a photo sensor in all parts except those used for writing.
In synchronization detection method, photoreception can be performed at one end of a scan line or both ends of the scan line. The latter method can be expected to considerably reduce non-coincidence of the beam spots in the main scanning direction as a standard for the write start timing is set at both ends of the scan line. However, in a color image forming apparatus producing color images, two photo sensors would be required per scanning optical system (for each color), increasing the number of components in terms of the photo sensors as well as electrical control boards for the photo sensors.
A technology thus is sought to enable adjustment of the write start timing using as few photo receptors as possible.
Again stability is desirable in the light beam used in detecting synchronization (hereinafter, “synchronization-detecting light”) as it is taken as a standard for the write start timing. However, even the synchronization-detecting light that reaches the photo sensor is affected by the deterioration of the optical performance of the scanning lens due to reduced tolerance, temperature variation, and the like.
In the optical scanning apparatus disclosed in Japanese Patent Application Laid-open No. 2002-98921, a notch is provided at an end of the scanning lens for the passage of the synchronization-detecting light, so that the synchronization-detecting light does not pass through the scanning lens and be adversely affected by the expansion and contraction of the scanning lens.
In the optical scanning apparatus disclosed in Japanese Patent Application Laid-open No. 3293345, a rib is provided for the passage of the synchronization-detecting light, again so that the synchronization-detecting light does not pass through the scanning lens and be adversely affected by the expansion and contraction of the scanning lens.
In an apparatus with optical writing as its principal functionality, the synchronous optical systems need to be placed where they will not interfere with the parts performing optical writing. This puts a constraint on the freedom in designing, especially with increasing demand for low-cost and compact optical scanning apparatus. Particularly, this constraint necessitates the length of the synchronous optical system path to be increased, leading to less than ideal conditions for photoreception.
A method is disclosed in Japanese Patent Application Laid-open No. H3-81720 wherein a synchronization mirror which forms a synchronization optical system and a synchronization detecting unit are coupled within a sub-scanning cross-section.
The synchronization-detecting light only reduces the shift of the beam spots of each color individually in the main scanning direction and by no means addresses the shift of the beam spots between different colors.
To obtain a high quality image, merely reducing the shift of the beam spot of each color individually in the main scanning direction is not enough, but it is also necessary for the optical scanning apparatus and the optical writing apparatus to be able to reduce the shift in the beam spots of different colors (hereinafter, “color shift”) in the main scanning direction, and not allow color shift due to temperature variation.
The layout constraint encountered in a quest to make the scanning optical systems compact cannot be bypassed only in the method disclosed in Japanese Patent Application Laid-open No. H3-81720.